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California's inter-agency Climate Action Team on Wednesday issued the first of 40 reports on impacts and adaptation, outlining what the state's residents must do to deal with the floods, erosion and other effects expected from rising sea levels.

Hundreds of thousands of people and billions of dollars of Golden State infrastructure and property would be at risk if ocean levels rose 55 inches by the end of the century, as computer models suggest, according to the report.

The group floated several radical proposals: limit coastal development in areas at risk from sea rise; consider phased abandonment of certain areas; halt federally subsidized insurance for property likely to be inundated; and require coastal structures to be built to adapt to climate change.

"Immediate action is needed," said Linda Adams, secretary for environmental protection. "It will cost significantly less to combat climate change than it will to maintain a business-as-usual approach."

Few topics are likely to be more contentious than coastal development. But along the state's 2,000-mile shoreline the effects would be acute, particularly in San Mateo and Orange counties, where more than 100,000 people would be affected, according to the 99-page state-commissioned report by the Oakland-based Pacific Institute.

Detailed maps of the coastline, published on the institute's website, show that residential neighborhoods in Venice and Marina del Rey could find themselves in a flood zone. Water could cover airports in San Francisco and Oakland, parts of the ports of Los Angeles and Long Beach, and large swaths of Huntington Beach and Newport Beach.

Roads, schools, hospitals, sewage plants and power plants may have to be relocated. More than 330 hazardous waste sites are at risk from floods.

"The rising sea level could be California's version of Hurricane Katrina," said Michael Woo, a Los Angeles planning commissioner and urban planning professor at USC. "Taxpayers and insurance ratepayers might question their responsibility to help homeowners and businesses which knowingly build in high-risk coastal areas."

California's far-reaching adaptation initiative reflects an emerging global consensus: Scientists can argue over how fast the Earth is heating up and diplomats can wrangle over emissions caps, but politicians must begin planning for the certainty of climate change.

Dozens of world-class scientists and economists, many from the University of California and state research institutes, are examining potential effects of warming on snowpacks, wildfires, crops and electricity demand.

Further reports will examine climate effects on hospital admissions, mortality rates, pollution and the habitats of the state's animals and plants. Dutch experts have been consulted on how to armor the coast with improved dikes and sea walls -- controversial measures that some experts contend will only increase erosion.

Detailed studies, now undergoing peer review, are to be released over the next month. Then the Climate Action Team is to send a comprehensive report to Gov. Arnold Schwarzenegger.

Sea levels along California have risen an average of about 8 inches in the past century. According to the Pacific Institute report, 260,000 Californians already live in flood zones, but are assumed to be protected by existing levees and sea walls. A rise of 1.4 meters (55 inches) would increase the population at risk to 480,000. Currently, 1,900 miles of roads and highways are at risk of flooding, which would grow to 3,500 miles under the sea level rise projections.

The report estimated that one adaptation strategy, armoring the coast with 1,100 miles of new or modified sea walls and levees, would cost at least $14 billion to construct, and another $1.4 billion a year to maintain.

The report's estimate of 1 to 1.4 meters of sea level rise by the end of the century was calculated using two scenarios envisioned by the United Nations' Intergovernmental Panel on Climate Change, a gathering of the world's top climate scientists. One scenario assumes countries will cut their emissions of planet-heating greenhouse gases, and another assumes a business-as-usual release level.

Despite more than a decade of warnings from scientists, global emissions continue to rise, fueled by rapid population growth and economic development in such nations as China and India. Unless greenhouse gases are cut significantly, Earth's temperature is expected to increase between 4 and 6 degrees Celsius by the end of the century, according to the U.N. panel.

As water warms due to rising air temperatures, it expands, causing the sea level to rise. But another major factor, the melting of the Greenland and Antarctic ice sheets, was unaccounted for in the U.N. panel's models because of uncertainty over effects and timing. Those models were designed in the mid-1990s.

Ice sheet melting has since accelerated. Dan Cayan, a researcher at the Scripps Institution of Oceanography and a lead scientist on the state's action plan, said the 55-inch estimate in the report is "probably conservative. . . . As temperature climbs, melting is going to proceed at a greater pace. It is not necessarily going to proceed linearly, in the same proportion as it did in the past, because melting begets more melting."

Low-income people will be disproportionately vulnerable to sea level rise, according to the report.

In Alameda County, 66,000 residents would be affected by flooding, of whom 60% are African American, Latino and Asian, the report said.

Mary Nichols, chairwoman of the state's Air Resources Board, which is charged with implementing a statewide plan to cut greenhouse gas emissions, called the sea level report "blunt but realistic."

"The recommendations are sensible: Defend what is worth protecting, move what can reasonably be moved, try to avoid doing further harm, consult affected communities, prepare to respond to emergencies."

Environmentalists hailed the report as a call to action.

"We can't pretend that the future will behave like the past," said Matt Vander Sluis of the Planning and Conservation League. "The ostrich has to take its head out of the sand or, in this case, it's going to be underwater."